1. Field of the Invention
The present invention relates to a method of increasing helical twisting power (HTP) in an optically active compound employed in a liquid crystal display device, an optically active compound, a liquid crystal composition containing the compound, and a liquid crystal display device using the same.
2. Description of Related Art
In liquid crystal materials for use in TN (twisted nematic) type, STN (super twisted nematic) type and phase transition type (cholesteric) devices, an optically active compound having a liquid crystal skeleton is added to induce a twisted orientation. It is known that a natural pitch P (μm) decreases when an additive concentration C (% by weight) of the optically active compound increases and that a good relationship of the product of P and C being constant is established in the case of low concentration C ranging from about 1 to several % by weight. The reciprocal of the relationship, i.e., a helical twisting power HTP (1/μm) (HTP=1/(P×0.01C) is used as a parameter for evaluation of a power for inducing the twisted orientation peculiar to the optically active compound.
Heretofore, as the optically active compound, a compound (product name: S-811) represented by the formula (VI-a):
a compound (product name: CB-15) represented by the formula (VI-b):
and a compound (product name: cholesteryl nonanate CN) represented by the formula (VI-c):
have commonly been used; however, they had small HTP values, for example, 10.5, 8.4 and 5.2 (as the HTP value, we use a value obtained by measuring at 25° C. after adding 1% by weight of the optically active compound to the following liquid crystal composition (XII-a)).
When using such an optically active compound having a small HTP value, it becomes necessary to add a large amount of the optically active compound to obtain a desired natural pitch, thereby causing problems such as increase in viscosity of the liquid crystal material and narrow temperature range of the liquid crystal. Thus, an optically active compound having a larger HTP value is required.
In the case of a cholesteric liquid crystal composition, since the pitch must be controlled within a range from about 0.1 to 2 μm so as to obtain a selective reflection wavelength in the infrared or visible range, the additive concentration of the optically active compound increases to 10 to 20% by weight. Since the upper temperature limit of the liquid crystal is drastically lowered when the additive concentration of the optically active compound increases, an optically active compound having a large HTP value has been required.
As a means for solving these problems, an optically active compound represented by the formula (VI-d):
(see Oiwa, Preliminary Manuscript of the 2001 Conference of the Japanese Liquid Crystal Society, 2001, p. 421–422), an optically active compound represented by the formula (VI-e):
(see Japanese Examined Patent Application, Second Publication No. Hei 7-33354 (claim 3)) and an optically active compound represented by the formula (VI-f):
(see Japanese Examined Patent Application, Second Publication No. Hei 5-52298 (claim 3)) are disclosed. These compounds have a structure having a phenyl group adjacent to an asymmetric carbon and have a large HTP value as compared with optically active compounds such as compounds represented by the formulas (VI-a), (VI-b) and (VI-c). However, the HTP value of these compounds is not sufficient to solve the problems described above, and therefore, a compound having a larger HTP value or a method of increasing the HTP value has been required.
A comparison in HTP between a structure having a substituent at the para-position of the end of these compounds and a structure having no substituent is made (see Koike, Preliminary Manuscript of the 1999 Conference of the Japanese Liquid Crystal Society, 1990, p. 220–221). The documents described above disclose that the HTP value decreases when the substituent is introduced at the para-position, and it was considered that the structure having a substituent at the end phenyl group is disadvantageous in view of the HPT value.